The present invention relates to alkylation of isoparaffins with olefins to produce branched chain paraffin hydrocarbons. More particularly, the present invention relates to the alkylation of isobutane with butylenes and propylene employing an alkylation catalyst comprising H.sub.2 SO.sub.4. Specifically, the present invention relates to an improved method for removing impurities from an isobutane charge stream to such an alkylation process, which impurities are reactive with sulfuric acid under alkylation conditions. The improved method of the present invention comprises treating an isobutane charge stream with spent, H.sub.2 SO.sub.4 containing alkylation catalyst to remove reactive impurities such as water, H.sub.2 S, mercaptans, etc. from the isobutane stream.
Alkylation of branched chain isoparaffin hydrocarbons with olefin hydrocarbons in the presence of an alkylation catalyst is well known. Processes for the alkylation of relatively low molecular weight isoparaffin hydrocarbons such as isobutane, isopentane, etc. with olefin hydrocarbons such as propylene and/or butylene to produce branched chain hydrocarbons suitable for use in motor fuel are widely practiced in commercial facilities.
Alkylation reactions of isobutane with butylene and/or propylene, and catalyzed by sulfuric acid containing catalyst, are carried out in the liquid phase under conditions such that a hydrocarbon phase comprising isobutane is maintained in intimate contact with a liquid catalyst phase comprising sulfuric acid. Alkylation reaction conditions include temperatures in the range of from about -20.degree. to 100.degree. F., pressures of from about 45 to 400 psia, and residence times of reactants under alkylating conditions of from about 30 seconds to about 60 minutes. Isobutane to olefin molar ratios employed may be from about 2:1 to about 25:1 and volume ratios of hydrocarbon phase to catalyst phase of from about 0.5:1 to about 3:1 may be employed.
Olefin charge stocks to such alkylation processes may not be completely pure, and may contain such impurities as butane, propane, and ethane. Such normal paraffin hydrocarbons are unaffected by the alkylation reaction. In continuous alkylation processes employing hydrocarbon recycle streams (primarily unreacted isobutane), such normal paraffin hydrocarbons must be removed from the alkylation process. Propane and ethane are generally removed from alkylation processes by venting as a gas stream, whereas n-butane is commonly removed by fractional distillation. It is desirable that a relatively high conversion of olefin charge be obtained in the alkylation reaction by obtaining high olefin conversion on a one-pass basis. Consequently, the ratio of isobutane to olefin is generally maintained sufficient to obtain substantially complete olefin conversion.
Isobutane charge may contain hydrocarbon impurities such as propane and normal butane, which are not converted in the alkylation process (as well as water, mercaptans, hydrogen sulfide, etc.). As isobutane is commonly employed in substantial molar excess to olefin charge in an alkylation process, it is common practice to recycle an isobutane-containing stream to the alkylation reaction. The hydrocarbon impurities, such as propane and normal butane, if allowed to accumulate in the isobutane recycle stream, would soon reach a concentration sufficient to interfere with the alkylation reaction by diluting the isobutane concentration in the hydrocarbon phase. Therefore, in commercial processes, the isobutane for recycle is commonly fractionated to separate such impurities as propane and normal butane therefrom.
Sulfuric acid containing about 1-7 wt. percent water is an effective catalyst to promote alkylation of isobutane with olefins. Such sulfuric acid may be used alone, or in combination with other catalysts such as flurosulfonic acid, and/or with catalyst promoters and surfactants. Catalyst containing sulfuric acid is added to the alkylation process whereupon it contacts olefin and isoparaffin hydrocarbons in the reaction zone. Olefin, being slightly soluble, mixes with H.sub.2 SO.sub.4 to form a liquid catalyst phase. Isobutane is substantially insoluble in such catalyst and forms a separate liquid phase. In the reaction zone, the two liquid phases are subjected to agitation to form an emulsion such that isoparaffin and olefin hydrocarbons are brought into intimate contact in the presence of the alkylation catalyst. Upon completion of the reaction step, the hydrocarbon phase (comprising alkylate and unreacted isobutane) and catalyst phase are separated by techniques well known in the art, such as for example, settling, coalescing, etc.
In commercial isobutane-olefin alkylation processes, an alkylation reaction effluent mixture is separated into a catalyst phase and a hydrocarbon phase by liquid-liquid separation means. The hydrocarbon phase comprises alkylated hydrocarbons, isobutane, and undesirable materials including ethane, propane, and normal butane which have entered the process mainly as components of the olefin and isobutane charge streams. As isobutane is used in the alkylation reaction in substantial molar excess, it is common practice to treat the hydrocarbon effluent from an alkylation reaction in a plurality of fractionation zones to separate a stream comprising isobutane which is recycled within the process to the alkylation reaction zone.
A major portion of the catalyst phase separated from an alkylation reaction effluent is commonly recycled to the alkylation reaction zone for use in alkylating additional amounts of isobutane with olefin. An alkylation catalyst containing sulfuric acid gradually loses its catalytic activity as it is recycled within an alkylation process. Accumulation of water, sulfonated polymeric hydrocarbons and other side reaction products within the catalyst phase serve to reduce the catalytic activity of the sulfuric acid. Consequently, a minor portion of the separated catalyst phase is withdrawn from the alkylation process as spent catalyst to remove by-products from the process. Fresh, make-up catalyst is added to the alkylation reaction zone to replace the amount of catalyst removed from the process as spent catalyst and to restore catalytic activity to the catalyst phase maintained in the alkylation reaction zone.
Many isobutane streams commonly available as charge stock for an alkylation process contain impurities in addition to such inert impurities as propane and normal butane. Impurities such as water, methylpropene, butadiene-1,3-propadiene, butyne-1, sulfur dioxide, hydrogen sulfide and methyl mercaptan are often found in isobutane streams available from petroleum refining processes. Such impurities as these react with sulfuric acid. The reaction of such impurities with sulfuric acid reduces the catalytic activity, or in some cases, such as the sulfur compounds, may destroy the catalytic effectiveness of sulfuric acid for alkylation of isobutane with olefins. Therefore, it is common practice to treat isobutane charge streams to remove such impurities prior to admitting such isobutane streams into the alkylation process. Treating methods such as caustic washing, adsorbing impurities upon adsorbents such as molecular sieves, silica-gel, etc. are well known in the prior art.
Water may be present in the isobutane as obtained from an outside source or may result from a treating step, such as caustic washing, employed to remove other reactive impurities from the isobutane. Treatment of isobutane with water adsorbers such as silica-gel, bauxite, molecular sieves, etc. is well known in the art. In such treating methods as described above for removing reactive impurities including water from the isobutane, the medium employed to treat the isobutane gradually losses its capacity for removing such impurities. Consequently, where treating methods such as caustic washing are employed, the caustic must be discarded and replaced with fresh caustic. Where treatment of isobutane with a solid adsorbent is employed, the adsorbent may be discarded or, in some events, regenerated.